This application is based on application No. 11-302945 filed in Japan, the content of which is hereby incorporated by reference.
The present invention relates to a zoom lens system. More specifically, the present invention relates to a zoom lens system suitable for use as a projection optical system for a projector such as a liquid crystal projector that projects an image on a liquid crystal panel onto a screen, and an imaging optical system for a digital video camera and a digital still camera.
As zoom lens systems for projection, the following are known: a three-unit zoom type of negative, positive, positive configuration in which first and second lens units are moved for zooming (U. S. Patent No. 5,914,820, etc.); and a four-unit zoom type of negative, positive, negative, positive configuration in which second and third lens units are moved for zooming (Japanese Laid-open Patent Application No. 10-20192, etc.).
In these zoom types, it cannot be said that sufficient size reduction is achieved while optical performance necessary for projection zoom lens systems is satisfied. For example, in the type in which the first lens unit is moved for zooming like the former three-unit zoom lens system, the diameter of the first lens unit is apt to be large and when a focusing method in which one lens unit is moved out for focusing is adopted, the mechanical structure of the first lens unit is also large in the direction of the diameter. In the latter four-unit zoom type, since the second lens unit and the third lens unit are moved in opposite directions for zooming, space for movement is necessary for each of the second and third lens units. Provision of space for movement for each of the second and third lens units results in an increase in overall length.
An object of the present invention is to provide an improved zoom lens system.
Another object of the present invention is to provide a high-performance and compact zoom lens system.
These objects are achieved by a zoom lens system performing zooming by moving at least two lens units, wherein a stationary lens unit having negative optical power is disposed on the most enlargement side and the lens units moved for zooming include no lens unit having negative optical power.